Electrode for photobattery

ABSTRACT

An electrode comprising an electrode material of the same type as electrode materials used in Li-ion batteries and a dye is provided. The electrode may further comprise a semiconductor material. The electrode is used in the manufacture of a battery that is rechargeable using light.

FIELD OF THE INVENTION

The invention relates generally to electrodes. More specifically, theinvention relates to an electrode comprising, in combination, anelectrode material of the same type as electrode materials used inLi-ion batteries and a light sensitive dye of the same type as lightsensitive dyes used in dye-sensitized solar cells (DSSC). A batterycomprising an electrode according to the invention may be charged usinglight.

BACKGROUND OF THE INVENTION

A lithium-ion battery may be summarily defined as follows: the batterycomprises a cathode material (for example LiFePO₄, LiCoO₂, FeS₂, V₂O₅,etc.), a lithium salt (for example LiPF₆, LiTFSl, LiClO₄, LMO, Li₂CO₃,etc.) dissolved in a liquid solvent or a polymer, and an anode material(for example graphite, LTO, etc.). In cases where the anode and/orcathode materials are not good current conductor, the material can becoated with carbon and/or be deposited on metallic substrates (forexample aluminum, copper, etc.).

Scheme 1 outlines the operation of a lithium-ion battery. Referencenumeral 10 depicts the copper cathode current collector, referencenumeral 11 depicts the lithium ions conductive electrolyte, andreference numeral 12 depicts the aluminum anode current collector.During operation of the battery, oxidation of the anode material leadsto de-intercalation of lithium ions, and simultaneously the cathodematerial undergoes a reduction reaction leading to intercalation of thelithium ions into its structure. Thereafter, the battery may be chargedby application of an external current. The external circuit createsmovement of the electrons from the cathode (which is in a reduced state)towards the anode. This leads to oxidation of the cathode material andthus de-lithiation restoring the lithium in the anode material.Following this process, the battery may be charged and discharged in athousand cycles.

A dye-sensitized solar cell (DSSC) may be summarily defined as follows:the system requires that at least one of its faces comprise a currentcollector which is transparent to light (Scheme 2, arrow 1′). Thetransparent current collector can be a metallic grid or a very thinlayer of a metal, a conductive polymer or a transparent substrate (glassor polymer) coated with a layer of a material which is transparent andconductive such as an oxide (for example FTO, ITO, Al-doped ZnO, Gaand/or Si, etc.), a conductive polymer (for example PEDOT:PSS, etc.) ormetallic grids.

The photosensitive layer in a DSSC (Scheme 2, arrow 2′) comprises alayer of a semiconductive material (for example TiO₂, ZnO, SnO₂,“core-shell”, etc.). The layer must be as much transparent as possibleand must enable the adsorption of the photosensitive dye. Typically, thephotosensitive dye comprises organometallic molecules. This includesdyes wherein molecules have pyridyl groups and ruthenium (for exampleindustrial dyes known as “N3”, “black dye”, “SJW-E1”, “N719”, etc.). Thephotosensitive dye may also comprise organic molecules only (for example“TA-St-CA”, etc.).

The electrolyte in a DSSC (Scheme 2, arrow 3′) may be a liquid, a gel ora solid. In any case, the electrolyte must comprise a sacrificial redoxcouple. Typically, the sacrificial redox couple is I₃ ⁻/I⁻. However,other redox couples may also be used (for example Br₃ ⁻/Br,SeCN⁻/(SeCN)₂, (SCN)₂/SCN⁻, Co³⁺/Co²⁺, etc.). A catalyst (for exampleplatinum, gold etc.) is generally used (Scheme 2, arrow 4′) in order toincrease the recombination speed of the sacrificial couple.

Finally, a DSSC generally comprises a current collector (Scheme 2, arrow5′). The current collector may be transparent such as the oneillustrated in Scheme, arrow 1′, or non-transparent.

FIG. 1 succinctly outlines the operation of a DSSC. Reference numeral 13depicts a semiconductor, reference numeral 14 depicts a dye, referencenumeral 15 depicts the electrolyte, reference numeral 16 depicts acounter-electrode made of conductive glass, and reference numeral 17depicts the external circuit.

In a DSSC, the flux of electrons is created by the excitation of thephotosensitive dye. Excitation is effected by light and by the fact thatthe lowest unoccupied molecular orbital (LUMO) of the dye has an energylevel higher than the energy level of the conduction band of thesemiconductor. Accordingly, electrons may be captured by thesemiconductor and then the current collector when they leave the exciteddye (S*). The dye is oxidized into S⁺ and immediately reacts with thesacrificial redox couple R/R⁻ according to the reaction S⁺+R⁻→S+R.Finally, the electron arriving at the counter-electrode through theexternal circuit serves in the recombination of the sacrificial redoxcouple. Given that the reactions occurring are governed by kinetics,electron extraction from the excited dye via the semiconductor then thecurrent collector must be faster than the natural relaxation of the dye,in order to obtain this reaction mechanism.

Extensive research aimed at improving the quality of batteries is beingconducted. A large part of this work relates to electrodes.

SUMMARY OF THE INVENTION

The inventors have designed and built an electrode which, when used in abattery, allows for the battery to be charged using light. The electrodeaccording to the invention comprises, in combination, an electrodematerial of the same type as electrode materials used in Li-ionbatteries and a photosensitive dye of the same type as photosensitivedyes used in dye-sensitized solar cells (DSSC). In an embodiment of theinvention, the electrode may further comprise a semiconductor material.A battery comprising the electrode according to the invention may becharged using light.

The invention thus relates to the following according to an aspectthereof:

-   (1) Electrode comprising an electrode material of the same type as    electrode materials used in Li-ion batteries and a photosensitive    dye.-   (2) Electrode according to item (1), further comprising a    semiconductor material.-   (3) Electrode comprising a cathode material of the same type as    cathode materials used in Li-ion batteries and a photosensitive dye.-   (4) Electrode according to item (3), further comprising a    semiconductor material.-   (5) Electrode comprising an anode material of the same type as anode    materials used in Li-ion batteries and a photosensitive dye.-   (6) Electrode according to item (5), further comprising a    semiconductor material.-   (7) Electrode according to item (3) or (4), wherein the cathode    material is an olivine type material.-   (8) Electrode according to item (3) or (4), wherein the cathode    material is LiFePO₄, LiCoO₂, FeS₂ or V₂O₅; preferably the cathode    material is LiFePO₄.-   (9) Electrode according to item (5) or (6), wherein the anode    material is metallic lithium, graphite, silicon, or a metal oxide    such as Fe₂O₃, TiO₂ and Li₄Ti₅O₁₂; preferably the anode material is    metallic lithium or graphite.-   (10) Electrode according to any one of items (1) to (9), wherein the    electrode material is coated with carbon.-   (11) Electrode according to any one of items (1) to (9), wherein the    electrode material comprises particles having a size below 1 μm;    preferably the size of the particles is below 0.1 μm.

(12) Electrode according to any one of items (2), (4) and (6), whereinthe semiconductor material is TiO₂, ZnO, SnO₂, a core-shell, or acombination thereof; preferably the semiconductor material is TiO₂.

-   (13) Electrode according to any one of items (2), (4) and (6),    wherein the semiconductor material comprises particles having a size    below 100 nm; preferably the size of the particles is below 30 nm.-   (14) Electrode according to any one of items (2), (4) and (6),    wherein the semiconductor material is pre-calcined.-   (15) Electrode according to any one of items (1) to (9), wherein the    photosensitive dye is N3, black dye, SJW-E1, N719, an organic    photosensitive dye such as TA-St-CA, or a combination thereof;    preferably the photosensitive dye is N719.-   (16) Electrode according to any one of items (1) to (9), further    comprising a solvent, a dispersant, a binder, or a combination    thereof.-   (17) Electrode according to item (16), wherein the solvent is    N-methyl-2-pyrrolidine (NMP), water, acetone, an alcohol such as    methanol, propanol and butanol, dimethylformamide (DMF), dimethyl    sulfoxide (DMSO), or a combination thereof; preferably the solvent    is water.-   (18) Electrode according to item (16), wherein the dispersant is    polyvinylidene difluoride (PVDF), a tension-active agent that does    not react with electrode materials such as Triton-X100, an alkyl    bromide ammonium salt such as tetraethylammonium bromide, an    alkylbenzyldimethylammonium halide such as an    alkylbenzyldimethylammonium bromide or halide, a glycol ester such    as glycol stearate, a glycerol ester, or a combination thereof;    preferably the dispersant is Triton-X100.-   (19) Electrode according to item (16), wherein the binder is    polyethylene glycol (PEG), polyvinylidene difluoride (PVDF),    polyvinyl acetate (PVA), or a combination thereof; preferably the    binder is polyethylene glycol (PEG).-   (20) Electrode material comprising, in combination, an electrode    material of the same type as electrode materials used in Li-ion    batteries and a photosensitive dye.-   (21) Electrode material according to item (20), further comprising a    semiconductor material.-   (22) Electrode material according to item (20) or (21), wherein the    photosensitive dye is anchored to the surface of the electrode    material particles.-   (23) Electrode material according to item (21), wherein the    photosensitive dye is anchored to the surface of the electrode    material particles and the semiconductor material particles.-   (24) Solid substrate having deposited thereon a material as defined    in any one of items (20) to (23); preferably the solid subtract is a    fluorine-doped tin oxide glass (FTO glass).-   (25) Method of manufacturing an electrode, comprising a step of    bringing into contact an electrode material of the same type as    electrode materials used in Li-ion batteries and a photosensitive    dye.-   (26) Method according to item (25), wherein the electrode material    is mixed with a semiconductor material prior to the contacting step.-   (27) Method of manufacturing an electrode, comprising the following    steps: (a) preparing a film comprising an electrode material of the    same type as electrode materials used in Li-ion batteries; and (b)    bringing into contact the film and a solution comprising a    photosensitive dye.-   (28) Method according to item (27), further comprising a preliminary    step of (a1) mixing the electrode material with a semiconductor    material prior to conducting step (a).-   (29) Method according to item (27) or (28), wherein step (a)    comprises depositing the electrode material on a solid substrate;    preferably the solid substrate is a fluorine-doped tin oxide glass    (FTO glass).-   (30) Method according to item (27) or (28), wherein step (a)    comprises depositing the mixture electrode material and    semiconductor material on a solid substrate; preferably the solid    substrate is a fluorine-doped tin oxide glass (FTO glass).-   (31) Method according to item (29) or (30), wherein depositing of    the material is carried out by the Doctor Blade method, by the    immersion withdrawal or dipping withdrawing method, by a serigraphy    method, by the spin-coating method, or a combination thereof;    preferably depositing of the material is carried out by the Doctor    Blade method or by the immersion withdrawal or dipping withdrawing    method.-   (32) Method according to item (27) or (28), wherein step (a)    comprises using a solvent; and the method further comprises, between    steps (a) and (b), a drying step followed by a cooling step.-   (33) Method according to item (32), wherein the drying step is    carried out at a temperature of about 400° C. and under inert    atmosphere, preferably under nitrogen atmosphere; and cooling is    carried out naturally until room temperature is reached.-   (34) Method according to item (27) or (28), wherein step (b)    comprises dipping the film into the solution comprising a    photosensitive dye.

(35) Method according to item (27) or (28), further comprising a dryingstep after step (b) followed a cooling step.

-   (36) Method according to item (35), wherein the drying step is    carried out at a temperature between room temperature and 120° C.    and under inert atmosphere; and cooling is carried out naturally    until room temperature is reached.-   (37) Battery using an electrode as defined in any one of items (1)    to (19).-   (38) Battery according to item (37), which is rechargeable using    light.-   (39) Use of an electrode as defined in any one of items (1) to (19),    in the manufacture of a battery.-   (40) Use of a material as defined in any one of items (20) to (23),    in the manufacture of a battery.-   (41) Use of a solid substrate as defined in item (24), in the    manufacture of a battery.-   (42) Battery manufactured subsequent to a use as defined in any one    of items (39) to (41), which is rechargeable using light.

Other objects, advantages and features of the present invention willbecome more apparent upon reading of the following non-restrictivedescription of specific embodiments thereof, given by way of exampleonly with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the appended drawings:

Scheme 1 succinctly describes the operation of a Li-ion battery.

Scheme 2 succinctly outlines a dye-sensitized solar cell (DSSC).

FIG. 1 succinctly outlines the operation of a DSSC.

Scheme 3 illustrates an assembly according to the invention.

FIG. 2 is a scanning electron micrograph (SEM) of titanium oxide powderpre-calcined at 400° C. (Example 1).

FIG. 3 is a scanning electron micrograph (SEM) of the surface of theelectroactive electrode (Example 1).

FIG. 4 is an X-ray diffractogram (XRD) of a raw electrode assembly ofExample 1.

FIG. 5 is a cyclic voltammetry (CV) of an electrode according to Example1, measured at 0.1 mV/s, 116 μA.h capacity for a surface of 2.7 cm².

FIG. 6 is an open circuit voltage (OCV) evolution during light exposureof the photo battery according to the invention (Example 1).

FIG. 7 is an X-ray diffractogram (XRD) of an electrode according toExample 1 after OCV.

FIG. 8 is a high resolution scanning electron micrograph (HRSEM) of thesurface of the film deposited on a fluorine-doped tin oxide glass (FTOglass) after annealing for 1 hour at 400° C. under a flux of nitrogen(Example 2).

FIG. 9 outlines the electrochemical characterizations for athree-electrode cell of the photoactive electrode according to Example 2in an electrolyte containing 0.3M LiTFSI in a EC-DEC (30-70 mass %)solution vs. Li, and with a Li⁺/Li reference: part A of the figure is anOCV evolution during light exposure; and part B is the CV of the film,the potential is achieved with a scanning speed of 0.1 mv/s yielding acapacity of 74 μA.h/cm² (Example 2).

FIG. 10 is a scanning electron micrograph (SEM) of hydrothermallyobtained LiFePO₄ crystals (Example 3).

FIG. 11 illustrates the OCV evolution during light exposure of the filmassembled as a three-electrode cell using an electrolyte consisting of1M LiPF₆ dissolved in EC-DEC (30-70 mass %) vs. Li, and with a Li⁺/Lireference, total duration ˜11 days (Example 3).

FIG. 12 illustrates measurement of the OCV evolution during lightexposure of a film consisting of submicron particles of LFP (lithiumferrophosphate or lithium iron phosphate (LiFePO₄)) and N719 dye. Thefilm was assembled as a three-electrode cell in LiPF₆ (1M) dissolved inEC-DEC (30-70 mass %) vs. Li, and using a Li⁺/Li reference (Example 4).

FIG. 13 is an X-ray diffractogram by grazing incident angle of Example4, before OCV.

FIG. 14 is a high resolution transmission electron micrograph (HRTEM) ofExample 4 before OCV. The inclusion is the Fourrier transformation ofthe image which proves that LiFePO₄ (LFP) of tryphilite structure wasinitially present.

FIG. 15 is an X-ray microgram by grazing incident angle of Example 4,after OCV.

FIG. 16 is a high resolution transmission electron micrograph (HRTEM) ofExample 4, after OCV. The inclusion is the Fourrier transformation ofthe image which proves that FePO₄ (LFP) of heterosite structure isformed after light exposure.

FIG. 17 is an X-ray photoelectron spectroscopy (XPS) spectrum of Example5, before OCV (part A of the figure) and after OCV (part B of thefigure).

FIG. 18 is a measurement of the OCV evolution of a film consisting ofsubmicron LFP particles and N719 dye. The film was kept in the dark andassembled as a three-electrode cell in LiPF₆ (1M) dissolved in EC-DEC(30-70 mass %) vs. Li, and using a Li⁺/Li reference (Example 5).

FIG. 19 is a measurement of the OCV evolution of a film consisting ofTiO₂-LFP and N719 dye. The film was kept in the dark and assembled as athree electrode cell in LiPF₆ (1M) dissolved in EC-DEC (30-70 massv%)vs. Li, and using a Li⁺/Li reference (Example 6).

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

As used herein, the expression “electrode material of the same type aselectrode materials used in Li-ion batteries” refers to materials havingcharacteristics that are similar to those of anode and cathode materialsused in Li-ion batteries (i.e., a material that reacts as an anode or acathode material in a Li-ion battery). More specifically, the expressionrefers to a “cathode material of the same type as cathode materials usedin Li-ion batteries” or an “anode material of the same type as anodematerials used in Li-ion batteries”.

As used herein, the expression “light sensitive dye of the same type aslight sensitive dyes used in dye-sensitized solar cells (DSSC)” or theexpression “photosensitive dye” refers to dyes having characteristicsthat are similar to those of dyes used in photo batteries (i.e., a dyethat reacts as a dye in a photo battery). More specifically, one or theother of the two expressions refers to a material wherein moleculesabsorb photons. As such at least one electron of the molecule moves fromthe highest occupied molecular energy (HOMO) to the lowest unoccupiedmolecular orbital (LUMO).

As used herein, the expression “semiconductor material” refers to amaterial having a bandgap below about 4 eV.

The inventors have designed and built an electrode which, when used in abattery, allows for the battery to be charged using light. The electrodeaccording to the invention comprises, in combination, an electrodematerial of the same type as electrode materials used in Li-ionbatteries and a photosensitive dye of the same type as photosensitivedyes used in dye-sensitized solar cells (DSSC). In an embodiment of theinvention, the electrode may further comprise a semiconductor material.A battery comprising the electrode according to the invention may becharged using light, instead of an external current.

In an embodiment of the invention, the cathode is obtained by combininga cathode material of the same type as cathode materials used in Li-ionbatteries and a light sensitive dye of the same type as light sensitivedyes used in DSSC. In a preferred embodiment, the combination maycomprise a semiconductor material.

The cathode material of the same type as cathode materials used inLi-ion batteries may be an olivine type material. In an embodiment ofthe invention, the cathode material may be LiFePO₄, LiCoO₂, FeS₂ orV₂O₅. In a preferred embodiment, the cathode material is LiFePO₄. Thecathode material of the invention may be coated with carbon.

In an embodiment of the invention, the anode is obtained by combining ananode material of the same type as anode materials used in Li-ionbatteries and a light sensitive dye of the same type as light sensitivedyes used in DSSC. In a preferred embodiment, the combination maycomprise a semiconductor material.

The anode material of the same type as anode materials used in Li-ionbatteries may be metallic lithium, graphite, silicon, or a metal oxidesuch as Fe₂O₃, TiO₂, Li₄Ti₅O₁₂, etc. In a preferred embodiment, theanode material may be metallic lithium or graphite. The anode materialaccording to the invention may be coated with carbon.

The light sensitive dye of the same type as light sensitive dyes used inDSSC may be N3, “black dye”, SJW-E1 or N719. In a preferred embodimentof the invention, the light sensitive dye is N719.

The semiconductor material may be TiO₂, ZnO, SnO₂ or “core-shell”. In apreferred embodiment of the invention, the semiconductor material isTiO₂.

The three components of the cathode according to the invention, namely,cathode material of the same type as cathode materials used in Li-ionbatteries, light sensitive dye of the same type as light sensitive dyesused in DSSC and semiconductor material, are in close contact. In apreferred embodiment, the cathode material and the semiconductormaterial may first be mixed together then the dye added.

The cathode material used is in the form of submicron particles or inthe form of hydrothermally obtained particles. The size of the particlesmay be below 1 μm. In a preferred embodiment of the invention, the sizeof the particles is below 0.1 μm.

The semiconductor material used is in the form of nanometric powder. Thesize of the particles may be below 100 nm. In a preferred embodiment ofthe invention, the size of the particles is below 30 nm. Thesemiconductor material may be in the form of a pre-calcined powder.

The dye used may be in liquid form, and in a preferred embodiment,addition of the dye may be performed by dipping into the liquid the filmcomprising the cathode material and the semiconductor material. In anembodiment of the invention, the film comprises the cathode material butnot the semiconductor material.

The mixture, cathode material semiconductor material may furthercomprise other agents such as solvents (for example water,N-methyl-2-pyrrolidone (NMP), etc.), binders (for example PEG, etc.),dispersants (for example Triton-X100, etc.).

The electrode material (Scheme 3, arrow 2) is deposited on a substratethat is transparent and conductive (Scheme 3, arrow 1). The substrate isboth transparent to light and electronically conductive, allowingmovement of electrons from or to an external circuit. In thisembodiment, the electrolyte (Scheme 3, arrow 3) is lithiated similarlyto electrolytes used in Li-ion batteries. Also, the electrolyte containsno sacrificial redox couple. The anode (Scheme 3, arrow 4) is of thesame type as anodes used in Li-ion batteries.

Similarly to DSSC, during light exposure, the photosensitive dye isexcited and may finally be oxidized, since at least one electron of themolecule moves to the LUMO which is of higher energy than the conductionband of the semiconductor. The semiconductor thus sends electronstowards the external circuit via the current collector. The cathodematerial of the battery or battery cathode material (BCM) which has beenreduced, acts as sacrificial redox couple similarly to sacrificial redoxcouples in DSSC. Oxidation of the reduced BCM, Li(BCM) is performedaccording to the following reaction:

Li(BCM)+S⁺→Li⁺+(BCM)+S

For the reaction to proceed spontaneously, it is necessary that thestandard potential of the redox couple of the dye, S⁺/S be superior tothe standard potential of sacrificial redox couple, Li⁺/Li(BCM).Typically in DSSC, the redox couple used is I₃ ⁻/I⁻ which has a standardpotential of 0.53V vs. ESH and which corresponds to 3.57V vs. Li⁺/Li.Accordingly, any cathode material used in a battery having a standardpotential below 3.6V vs. Li⁺/Li may be oxidized when the material iscombined with a photosensitive dye of the same type as dyes used inDSSC. In this case, the battery cathode material in reduced state,Li(BCM) may be oxidized into BCM replacing the sacrificial redox coupleI⁻/I₃ ⁻ of DSSC. In a case where a cathode material is used in a batteryhaving a standard potential higher than 3.6V vs. Li⁺/Li, synthesisand/or use of specific dyes, similarly to DSSC using the redox coupleCo³⁺/Co²⁺, may be contemplated.

Lithium ions released following the reaction at the cathode are reducedat the anode by electrons from the external circuit. Accordingly, it ispossible to obtain oxidation of the reduced cathode material with noexternal current.

EXAMPLE 1

Composition of the Cathode Material (Paste):

-   -   TiO₂ in nanomeric powder form and pre-calcined at 400° C./1 h        (FIG. 2): 0.3 g    -   Submicron LiFePO₄ particles: 0.3 g    -   PVDF: 0.06 g    -   NMP: 3 mL

A wet film of the above paste was deposited on a FTO glass substrate(Cytodiagnostics TEC 7, 6-8Ω) by the Doctor Blade method (3 mils). Thefilm was annealed under a nitrogen flux according to the followingprocedure: the temperature is brought to 400° C. within 1 hour andmaintained at 400° C. during 1 hour followed by natural cooling untilroom temperature is reached.

The film obtained was dipped into an aqueous solution containingphotosensitive dye N719 (4×10⁻⁴M) during 24 hours. Dye was anchored tothe surface of the cathode material particles. Samples of the film weredried at 50° C. under vacuum during 48 hours (FIG. 3).

X-ray diffraction (XRD) diagram of the photoactive raw electrodeassembly (FIG. 4) shows the presence of tin oxide from the FTO glass,anatase and brookite phases from titanium oxide, and tryphilite phasefrom LiFePO₄. Peak distribution indicates that no heterosite FePO₄ phaseis present.

The photoactive electrode was assembled in a three-electrodeconfiguration, in LiPF₆ solution (1M) dissolved in EC-DEC-VC aselectrolyte vs. Li with a Li⁺/Li reference.

Electrochemical characteristics of these electrodes were measured bycyclic voltammetry (CV) with a scanning speed of 0.1 mV/s and a voltagerange from 2.5V to 4V with regard to lithium (FIG. 5). The CV wascompleted at a potential of 2.5V. The CV obtained is representative of afilm consisting of submicron LFP, given its oxidation potential at 3.46Vand its reduction potential at 3.4V.

The open circuit voltage (OCV) of the sample kept in the three-electrodecell and under the light hood was registered (FIG. 6) once the CV wascompleted at a 2.5V potential. As expected, a rapid increase of the OCVfrom 2.5V to 3.4V within 1 hour was obtained. Thereafter, the OCVremained stable during 24 hours at a potential between 3.43V and 3.44V.This stability allowed for the battery to be charged. At the end of theOCV, the voltage of the battery was increased from 3.44V to 3.65V. Thisincrease is due to the charging of the battery or photo battery asexpected.

After the OCV, the sample was again analyzed by X-ray diffraction (XRD)(FIG. 7). The main peaks correspond to phases observed in FIG. 4,namely, casserite (tin oxide), brookite and anatase. However, some peakchanges can be seen in the lower angle region (2θ<30°). These changesare probably due to the fact that part of phosphate lithium iron hasbeen oxidized to phosphate iron. It is not possible to assess from thediffractogram whether all triphilite LiFePO₄ was converted to heterositeFePO₄, however, the main part was converted.

An estimated mass ratio Fe/Ti at various locations of the film wasdetermined. The ratios were all between 1.49 and 2.02. It should benoted that this ratio is around 0.59 for a homogenous mixture comprising50-50 mass % LiFePO₄-TiO₂. The difference is probably due to the factthat TiO₂ particles were not well dispersed in the NMP (part of thetitanium oxide remained at the bottom of the pot). A lack of titaniumoxide may have an impact on the performance of the photo battery.Accordingly, NMP may be replaced by other solvents such as water forexample.

EXAMPLE 2

Composition of the Cathode Material (Paste):

-   -   TiO₂ in nanomeric powder form and pre-calcined at 400° C./1 h:        0.4 g    -   Submicron LiFePO₄ particles: 0.3 g    -   PEG as binder: 0.1 g    -   Water: 2 mL

A wet film of the above paste was deposited on a FTO glass substrate(Cytodiagnostics TEC 7, 6-8Ω) by the Doctor Blade method (3 mils). Thefilm was annealed under a nitrogen flux according to the followingprocedure: the temperature is brought to 400° C. within 1 hour andmaintained at 400° C. during 1 hour followed by natural cooling untilroom temperature is reached.

The film obtained was dipped into an aqueous solution containingphotosensitive dye N719 (4×10⁻⁴M) during 24 hours. Dye was anchored tothe surface of the cathode material particles. Samples of the film weredried at 50° C. under vacuum during 24 hours.

High resolution scanning electron microscopy (HRSEM) and energydispersive spectrometry (EDS) analyses were conducted in order to assessthe particles morphology and the mass ratio Fe/Ti. This sample containsno agglomerate having a needle form, contrary to Example 1 (FIG. 8). Amass ratio Fe/Ti of about 0.28 was measured by EDS. It should be notedthat the expected mass ratio for this paste was 0.44. This means thatthe amount of LiFePO₄was lower. Agglomerates having a needle form inExample 1 are probably due to an aggregation of nanomeric LiFePO₄particles.

The photo oxidation process was monitored by OCV (FIG. 9) and measuredduring light exposure. In this case, the photo oxidation is faster thanin Example 1, since a 4.2V voltage in less than 1 hour was measured.This improvement is probably due to a better mixture between LiFePO₄ andTiO₂. In addition, a CV test of the film shows that the film is reducedat a potential between 2.8-3.0V and is oxidized at a potential between3-3.2V. This is in conformity with the submicron size of the crystals.

EXAMPLE 3

Composition of the Cathode Material (Paste):

-   -   TiO₂ in nanomeric powder form and pre-calcined at 400° C./1 h: 5        g    -   LiFePO₄ particles hydrothermally obtained (FIG. 10): 5 g    -   Triton-X100 as binder: 0.3 mL    -   Water: 112 mL

This paste was deposited on a FTO glass substrate (Cytodiagnostics TEC7, 6-8Ω) by the method called “immersion withdrawal” or “dippingwithdrawing”. The film was annealed under a nitrogen flux according tothe following procedure: the temperature is brought to 400° C. within 1hour and maintained at 400° C. during 1 hour followed by natural coolinguntil room temperature is reached. The film obtained was dipped into anaqueous solution containing photosensitive dye N719 (4×10⁻⁴M) during 24hours. Dye was anchored to the surface of the cathode materialparticles. Samples of the film were dried at 50° C. under vacuum during24 hours.

OCV of the film during light exposure was then conducted and monitored:an increase of the potential from 3.45V to 3.65V in 11 days wasobserved. Comparing to Example 2, this photo-oxidation was slower. Thisis probably due to the fact that LiFePO₄ particles of the film are farbigger than the particles in Example 2.

EXAMPLE 4

Composition of the Cathode Material (Paste):

-   -   Submicron LiFePO₄ particles: 5 g    -   Triton-X100 as binder: 0.15 mL    -   Water: 50 mL

This paste was deposited on a FTO glass substrate (Cytodiagnostics TEC7, 6-8Ω) by the method called “immersion withdrawal” or “dippingwithdrawing”. The film was annealed under a nitrogen flux according tothe following procedure: the temperature is brought to 400° C. within 1hour and maintained at 400° C. during 1 hour, followed by naturalcooling until room temperature is reached. The film obtained was dippedinto an aqueous solution containing photosensitive dye N719 (4×10⁻⁴M)during 24 hours. Dye was anchored to the surface of the cathode materialparticles. Samples of the film were dried at 50° C. under vacuum during24 hours.

OCV of the film under light exposure was then conducted and monitored(FIG. 12): an increase of the potential from 3.4V to 3.75V in 21 dayswas observed.

FIG. 13 and FIG. 14 are, respectively, X-ray diffractogram (XRD) andhigh resolution transmission electron micrographs (HRTEM) performedprior to photo-oxidation of the film. It can be seen from these figuresthat before photo-oxidation, the electrode material consists exclusivelyof LiFePO₄ crystals having triphilite structure. Conversion oftriphilite LiFePO₄ to heterosite FePO₄ obtained by photo-oxidation ofthe film due to the presence of the photosensitive dye can be seen onthe XRD diffractogram and the HRTEM micrographs in FIG. 15 and FIG. 16.

FIG. 17 represents XPS measurements performed before (A) and after (B)photo-oxidation. A peak increase for Fe³⁺ species is observed afterphoto-oxidation. This is in conformity with the conversion of theLiFePO₄ triphilite phase to heterosite FePO₄. This transformation ofFe²⁺ from LiFePO₄ into Fe³⁺ of FePO₄ may be monitored by the strongreduction of peaks located at ˜709 and ˜723 eV both due to Fe²⁺ speciesand the increase of peaks located at ˜712 and ˜723 eV due to Fe³⁺species.

The characterization methods in this example show that photo-oxidationof LiFePO₄ to FePO₄ is possible with no addition of a semiconductor suchas TiO₂. However, it appears that use of a semiconductor allows for anincrease of the rate of the photo-oxidation.

EXAMPLE 5

Composition of the Cathode Material (Paste):

-   -   Submicron LiFePO₄ particles: 5 g    -   Triton-X100 as binder: 0.15 mL    -   Water: 50 mL

This paste was deposited on a FTO glass substrate (Cytodiagnostics TEC7, 6-8Ω) by the method called “immersion withdrawal” or “dippingwithdrawing”. The film was annealed under a nitrogen flux according tothe following procedure: the temperature is brought to 400° C. within 1hour and maintained at 400° C. during 1 hour followed by natural coolinguntil room temperature is reached. The film obtained was dipped into anaqueous solution containing photosensitive dye N719 (4×10⁻⁴M) during 24hours. Dye was anchored to the surface of the cathode materialparticles. Samples of the film were dried at 50° C. under vacuum during24 hours.

The film was kept in the dark during OCV measurement. After 23 days, thepotential reached a plateau at 3.4V vs. Li⁺/Li (FIG. 18). Contrary toExample 4, no increase of the potential was observed. This example showsthe requirement of photosensitive dye to be present in order to removelithium from LiFePO₄. Also, it appears that light is required.

EXAMPLE 6

Composition of the Cathode Material (Paste):

-   -   TiO₂ in nanomeric powder form and pre-calcined at 400° C./1 h: 5        g    -   Submicron LiFePO₄ particles: 5 g    -   Triton-X100 as binder: 0.3 mL    -   Water: 112 mL

This paste was deposited on a FTO glass substrate (Cytodiagnostics TEC7, 6-8Ω) by the method called “immersion withdrawal” or “dippingwithdrawing”. The film was annealed under a nitrogen flux according tothe following procedure: the temperature is brought to 400° C. within 1hour and maintained at 400° C. during 1 hour followed by natural coolinguntil room temperature is reached. The film obtained was dipped into anaqueous solution containing photosensitive dye N719 (4×10⁻⁴M) during 24hours. Dye was anchored to the surface of the cathode materialparticles. Samples of the film were dried at 50° C. under vacuum during24 hours.

The film was kept in the dark during OCV measurement. Contrary toExample 2 wherein the system reached a potential of 4.2V in less than 1hour, the potential reached a plateau of 3.4V with no increase for 14days (FIG. 19). This example shows that despite the presence of asemiconductor used to improve the removal of lithium in LiFePO₄, lightwas required for the reaction to take place.

Although the present invention has been described hereinabove by way ofspecific embodiments thereof, it may be modified, without departing fromthe spirit and nature of the subject invention as defined in theappended claims.

The present description refers to a number of documents, the content ofwhich is herein incorporated by reference in their entirety.

REFERENCES

-   B. O'Reagan and M. Grätzel, Nature (1991) 353, 737-740.

1. Electrode comprising an electrode material of the same type aselectrode materials used in Li-ion batteries and a photosensitive dye.2. Electrode according to claim 1, further comprising a semiconductormaterial.
 3. Electrode comprising a cathode material and/or an anodematerial, and a photosensitive dye, wherein the cathode and anodematerials are of the same type as cathode and anode materials used inLi-ion batteries. 4-6. (canceled)
 7. Electrode according to claim 3,wherein the cathode material is an olivine type material selected fromLiFePO₄ and LiCoO₂, or FeS₂, or V₂O₅.
 8. (canceled)
 9. Electrodeaccording to claim 3, wherein the anode material is metallic lithium,graphite, silicon, or a metal oxide.
 10. Electrode according to claim 1,wherein the electrode material is coated with carbon.
 11. Electrodeaccording to claim 1, wherein the electrode material comprises particleshaving a size below 1 μm or a size below 0.1 μm.
 12. Electrode accordingto claim 2, wherein the semiconductor material is TiO₂, ZnO, SnO₂, acore-shell, or a combination thereof.
 13. Electrode according to claim2, wherein the semiconductor material comprises particles having a sizebelow 100 nm or a size below 30 nm.
 14. Electrode according to claim 2,wherein the semiconductor material is pre-calcined.
 15. Electrodeaccording to claim 1, wherein the photosensitive dye is N3, black dye,SJW-E1, N719, an organic photosensitive dye, or a combination thereof.16. Electrode according to claim 1, further comprising a solvent, adispersant, a binder, or a combination thereof.
 17. Electrode accordingto claim 16, wherein: the solvent is N-methyl-2-pyrrolidine (NMP),water, acetone, an alcohol, dimethylformamide (DMF), dimethyl sulfoxide(DMSO), or a combination thereof; and/or the dispersant ispolvvinvlidene difluoride (PVDF), a tension-active agent that does notreact with electrode materials, an alkyl bromide ammonium salt, analkylbenzyldimethylammonium halide, a glycol ester, a glycerol ester, ora combination thereof; and/or the binder is polyethylene glycol (PEG),polvvinylidene difluoride (PVDF), polyvinyl acetate (PVA) or acombination thereof. 18-21. (canceled)
 22. Electrode material accordingto claim 1, wherein the photosensitive dye is anchored to the surface ofthe electrode material particles.
 23. Electrode material according toclaim 2, wherein the photosensitive dye is anchored to the surface ofthe electrode material particles and the semiconductor materialparticles.
 24. Solid substrate having deposited thereon a material asdefined in claim 1; optionally the solid subtract is a fluorine-dopedtin oxide glass (FTO glass).
 25. (canceled)
 26. (canceled)
 27. Method ofmanufacturing an electrode, comprising the following steps: (a)preparing a film comprising an electrode material of the same type aselectrode materials used in Li-ion batteries; and (b) bringing intocontact the film and a solution comprising a photosensitive dye. 28.Method according to claim 27, further comprising a preliminary step of(a1) mixing the electrode material with a semiconductor material priorto conducting step (a).
 29. Method according to claim 27, wherein step(a) comprises depositing the electrode material on a solid substrate,optionally the solid substrate is a fluorine-doped tin oxide glass (FTOglass).
 30. Method according to claim 28, wherein step (a) comprisesdepositing the mixture electrode material and semiconductor material ona solid substrate; optionally the solid substrate is a fluorine-dopedtin oxide glass (FTO glass).
 31. Method according to claim 29, whereindepositing of the material is carried out by the Doctor Blade method, bythe immersion withdrawal or dipping withdrawing method, by a serigraphymethod, by the spin-coating method, or a combination thereof.
 32. Methodaccording to claim 27, wherein step (a) comprises using a solvent; andthe method further comprises, between steps (a) and (b), a drying stepfollowed by a cooling step; optionally the drying step is carried out ata temperature of about 400° C. and under inert atmosphere, and coolingis carried out naturally until room temperature is reached. 33.(canceled)
 34. Method according to claim 27, wherein step (b) comprisesdipping the film into the solution comprising a photosensitive dye. 35.Method according to claim 27, further comprising a drying step afterstep (b) followed a cooling step; optionally the drying step is carriedout at a temperature between room temperature and 120° C. and underinert atmosphere; and cooling is carried out naturally until roomtemperature is reached.
 36. (canceled)
 37. Battery using an electrode asdefined in claim
 1. 38. Battery according to claim 37, which isrechargeable using light. 39-42. (canceled)